Transglutaminase is a ubiquitous intracellular enzyme that covalently cross-links proteins. Studies with fluorescent derivatives of polypeptide hormones, insulin and Epidermal growth factor (EGF) and the serum protein alpha 2-macroglobulin have indicated that transglutaminase is essential for receptor mediated endocytosis of these proteins. Inhibitioan of transglutaminase potentiates the mitogenic activity of EGF suggesting the enzyme may play a role in regulating the responsiveness of cells to hormones and growth factors. The proposed studies are designed to characterize in cultured cells the transglutaminase important for hormone internalization. We plan to study the subcellular localization of the enzyme and its relationship to liver transglutaminase. We will also study the activation of fibroblast transglutaminase to establish whether hormones can activate the enzyme and what the mechanism of activation is. Lastly we will identify the substrates of transglutaminase and using affinity chromatography attempt to purify and characterize them. From these studies we hope to gain a better understanding of how transglutaminase participates in hormone internalization and how the enzyme may contribute to states of normal and abnormal growth control.